1. Field of the Invention
The present invention relates to transparent, fixed tint glass lenses and, more particularly, relates to a fixed tint lens derived from an alkali silicate composition containing high concentrations of colorants and thicknesses of between about 1.0 mm to about 1.8 min.
2. Description Of The Art
Transparent fixed tint lenses have been marketed in sunglasses for many years by many different companies. For example, the assignee of the present application has marketed 2.2 mm thick glass lenses sold under the registered trademark G-15 since the 1940's. These lenses provide the true color vision of a neutral gray lens and are typically air-tempered to improve their impact resistance. The G-15 lenses are known for their unique color which provides true color vision essentially equivalent to a perfect neutral lens while, at the same time, avoiding the flat and dull appearance of a perfectly neutral lens. This is achieved by departing slightly from a neutral spectral transmission towards a slight green. The slight green transmission is due to a blue and red absorption slightly beyond the perfectly neutral spectral absorption. This absorption leads to a higher contrast or a "livelier" view as compared to the view through a perfectly neutral lens. Many sunglass wearers prefer this lively view as opposed to the flat view often associated with conventional neutral gray lenses. It is the precise control of the colorants in the glass, namely the Group VIII metal oxides such as the iron, cobalt and nickel oxides, which allow the perfect balance of neutrality and green color. As a result, the lively gray G-15 lens has been commercially successful and millions of sunglasses are sold each year containing these lenses.
While the 2.2 mm thick glass lenses have been commercially successful, there are some drawbacks associated with glass lenses in general. Conventional glass sunglass lenses typically have a thickness from 2.0 mm to greater than 2.2 mm thick. The weight of such glass lenses is considered a disadvantage by some sunglass wearers. One approach to address this weight concern has been to substitute plastic lenses for the glass lenses since the weight of comparable plastic lenses are substantially less than their glass counterparts. However, this approach has not proved to be entirely satisfactory since the plastic lenses are much more prone to scratching, have imperfect clarity, and occasionally have unstable colorants. Attempts to produce a thinner glass lens having substantially the same optical properties as the present 2.2 mm thick G-15 lens utilizing conventional Beer's Law to scale the colorant concentrations was not successful. This approach lead to glass lenses having reduced transmittance (too dark) and a dominant wave length shifted toward the yellow-green (too yellow-green).
Glass lenses, as disclosed herein, have now been developed which are thinner than conventional lenses, have the associated reduced weight, and are suitable for use as sunglass lenses or, in certain applications, as ophthalmic lenses. However, it has been found that these lenses must have precise concentrations of colorants and oxidizing agents to achieve desirable visual characteristics. The lenses of this invention have a thickness of between about 1.0 mm and about 1.8 mm and are derived from an alkali silicate composition containing at least 6.2% by weight iron oxides, at least 0.03% by weight cobalt oxide and at least 0.12% by weight nickel oxide and an oxidizing agent. These lenses preferably have a transmittance of not more than 1% of ultraviolet radiation at 380 nm. While many sunglasses have been marketed with neutral gray, fixed tint glass lenses, none of these lenses have optical properties or compositions similar to the present lenses and have a thickness of between about 1.0 mm and about 1.8 min.
Accordingly, the present invention is directed to thin glass lenses suitable for use in sunglasses and fixed tint ophthalmic lenses which exhibit a lively gray, fixed tint transmittance, unlike those previously available.